The use of microwave energy to cook a variety of foodstuffs to an edible condition is quick and convenient. However, some foodstuffs require crispening or browning to be acceptable for consumption, which is not possible with conventional microwave cooking.
It is known from U.S. Pat. No. 4,641,005 (Seiferth), assigned to James River Corporation, that it is possible to generate thermal energy from a thin metallic film (microwave susceptor) upon exposure thereof to microwave radiation and this effect has been used in a variety of packaging structures to achieve cooking of foodstuffs with microwave energy, including achieving crispening and browning, for example, of pizza crust.
Some food products which are to be cooked by microwave energy are in the form of an outer pastry dough shell and an inner filling. An example is an apple turnover. One problem which has arisen when packages employing thin metal films to generate thermal energy to obtain crispening and browning of such products, is that there is a considerable moisture loss from the filling and sometime a spilling of filling as the shell splits open, leading to an unsatisfactory product.
In addition, certain foodstuffs are difficult to brown and crispen satisfactorily. For example, while it is possible to improve the cooking of pot pies when compared to conventional oven-cooked pot pies, by the employment of microwave energy and multiple thin films of electroconductive material in the bottom of the dish, as described in my copending U.S. patent application Ser. No. 442,153 filed Nov. 28, 1989 ("Pot Pie Dish"), the disclosure of which is incorporated herein by reference, nevertheless the resulting product does not exhibit an ideal degree of browning.
Attempts have been made to improve the overall uniformity of heating which results when thin metal film microwave susceptors are exposed to microwave radiation. One such proposal is contained in U.S. Pat. No. 4,927,991 (Wendt), assigned to The Pillsbury Company, which describes the employment of a microwave-reflective grid in combination with a thin metal film microwave susceptor. The structure is stated to achieve a more uniformly heated foodstuff by controlling surface heating and microwave transmittance.
Another approach to the microwave cooking of foodstuffs is described in U.S. Pat. No. 3,845,266 (Derby), assigned to Raytheon Company. This patent describes a utensil for microwave cooking, which is intended to be reusable in a microwave oven and is illustrated, in one embodiment, as taking the form of a slotted rigid stainless steel plate. The slotted nature of the stainless steel plate is said to achieve browning and searing of foodstuff in contact with it in a microwave oven. The stainless steel plate sits on a member of microwave transparent material, such as glass, in the cavity of a microwave oven to effect such heating.
It also has been previously suggested from U.S. Pat. No. 4,230,924 (Brastad et al) to provide microwave energy generated browning of a foodstuff from a food package which includes a flexible wrapping sheet of polymeric film having a flexible metal coating, which either may be relatively thin film or relatively thick foil and which, in either case, is subdivided into a number of individual metallic islands in the form of squares. It has been found that, while some thermal energy generation is achieved by such structures, both with the relatively thin film and the relatively thick foil, little or no shielding of microwave energy is achieved using the described relatively thick foil structure. In this latter prior art, the metal is provided in the form of discrete islands which are separated one from another, and hence the metallized portion of the substrate is discontinuous in character.
Further, there have been a variety of proposals to moderate the proportion of incident microwave energy reaching a foodstuff by using perforated aluminum foil. For example, U.S. Pat. Nos. 4,144,438, 4,196,331, 4,204,105 and 4,268,738, all assigned to The Procter & Gamble Company, disclose a microwave cooking bag formed from a laminate of two outer thermoplastic films sandwiching a perforated aluminum foil having a series of large circular apertures therethrough. While this arrangement may be useful in moderating the microwave energy entering the foodstuff, these openings are not of a size or shape which permits the generation of thermal energy, so that no surface browning can result.
Similarly, U.S. Pat. No. 3,219,460 (Brown), U.S. Pat. No. 3,615,713 (Stevenson), U.S. Pat. Nos. 3,985,992, 4,013,798 and 4,081,646 (Goltsos) describe T.V. dinner trays intended for use for microwave cooking of such foods, in which the lid is provided with apertures of varying dimension through microwave opaque materials incorporated into the lid structure to control the flow of microwave energy to the different food products in the tray. Again, the apertures are not of a size or shape to permit the generation of thermal energy.
In my prior U.S. patent application Ser. No. 650,246 filed Feb. 4, 1991 "(now U.S. Pat. No. 5,117,078)", assigned to the assignee hereof and of which is incorporated herein by reference, I have described an improved structure for the generation of thermal energy in a selected and controlled manner using flexible normally microwave-opaque electroconductive materials, for example, aluminum foil.
As described therein, a plurality of elongate apertures of appropriate dimensions is formed in the flexible electroconductive material, which results in the generation of thermal energy in the region of the apertures upon exposure of the flexible electroconductive material to microwave radiation. For the purpose of incorporation of the layer of flexible electroconductive material into a packaging structure, the layer of flexible normally microwave-opaque electroconductive material is supported on and is in adhered structural relationship with a substrate layer of microwave energy transparent material. The plurality of elongate apertures is sized and arranged in this prior art structure to generate sufficient thermal energy to effect a desired surface browning of the foodstuff while permitting sufficient microwave energy to penetrate the layer of flexible electroconductive material through the plurality of apertures into the foodstuffs to effect a desired degree of dielectric heating of the foodstuff, whereby the foodstuff may be provided in an edible condition.
This arrangement enables a much greater degree of control to be achieved over the microwave cooking of food products which are comprised of component parts which require different degrees of cooking, and, in particular those that require outer crispening or browning and yet may suffer from moisture loss, which may lead to some sogginess of product, if over-exposed to microwave energy.
The degree of thermal energy generation which can be achieved from this prior art structure is limited and, in the case of some foodstuffs, such as pre-cooked meat products, such as hot dogs, may be insufficient to provide the desired outer browning or searing effect, which providing a satisfactorily reheated product.